Zinc Ferrite Thin Film, Method for Manufacturing the Same and Application Thereof

ABSTRACT

Electrochemical methods for manufacturing a zinc ferrite (ZnFe 2 O 4 ) thin film include preparing an electrodeposition solution and forming the zinc ferrite thin film on a conductive substrate under suitable conditions. The electrodeposition solution includes about 10 −2  M to about 10 −1  M zinc nitrate aqueous solution and about 10 −3  M to about 10 −2  M ferric nitrate aqueous solution.

RELATED APPLICATIONS

This is a divisional application of patent application Ser. No.12/474,378 filed on May 29, 2009, now allowed. The prior applicationSer. No. 12/474,378 claims the benefit of Taiwan Patent Application97149473, filed Dec. 18, 2008, the disclosures of which are incorporatedherein by reference in their entirety.

BACKGROUND

1. Field of Invention

The present invention relates to a zinc ferrite thin film and methodsfor manufacturing the same. More particularly, the present inventionrelates to electrochemical methods for manufacturing a zinc ferrite thinfilm.

2. Description of Related Art

Nano-sized zinc ferrite (zinc ferrite, ZnFe₂O₄) can be used assemiconductor material and photocatalyst material since it is capable ofphoto-electro conversion. As comparing with titanium dioxide with wideband gap (3.2 eV for anatase and 3.0 eV for rutile), nano-sized zincferrite has narrow band gap of about 1.9 eV and shows visible lightabsorption. Thus, nano-sized zinc ferrite has higher efficiency ofsunlight utilization than neat titanium dioxide.

Generally, zinc ferrite can be formed on a substrate by a reactivesputtering technique and then annealed at a temperature of at least 500°C. to obtain the zinc ferrite thin film. Alternatively, powders ofnano-sized zinc ferrite can be annealed at a temperature of at least500° C. to obtain the zinc ferrite thin film. Methods for preparing saidnano-sized zinc ferrite powders include sol-gel process, hightemperature annealing process, and shock wave compress technique, etc.

Known methods for manufacturing the zinc ferrite thin film usuallyinvolves elaborate and costly apparatus and complicated process.Furthermore, said high annealing temperature limits the option of thematerial of the substrate and thus limits the application of the zincferrite thin film.

In view of the foregoing, there is a need to provide a method formanufacturing a zinc ferrite thin film, said method could employ lesscomplicated apparatus and process and utilize lower annealingtemperature than prior art does.

SUMMARY

The following presents a simplified summary of the disclosure in orderto provide a basic understanding to the reader. This summary is not anextensive overview of the disclosure and it does not identifykey/critical elements of the present invention or delineate the scope ofthe present invention. Its sole purpose is to present some conceptsdisclosed herein in a simplified form as a prelude to the more detaileddescription that is presented later.

In one aspect, the present invention is directed to electrochemicalmethods for manufacturing a zinc ferrite thin film. As comparing withreactive sputtering or other known methods for manufacturing zincferrite thin film, the electrochemical method for manufacturing zincferrite thin film could employ less complicated apparatus and processand utilize lower annealing temperature than prior art does.Furthermore, according to embodiments of the present invention, zincferrite thin film could be obtained without the annealing step.

According to embodiments of the present invention, the electrochemicalmethod for manufacturing zinc ferrite thin film includes the procedureof preparing an electrodeposition solution, immersing a conductivesubstrate in the electrodeposition solution and electrodepositing thezinc ferrite thin film on the conductive substrate under suitableparameters, and drying the zinc ferrite thin film with a dryingtemperature of about 15-40° C. and a relative humidity of at least about75%.

According to one embodiment of the present invention, theelectrochemical method for manufacturing zinc ferrite thin film includesthe procedure of preparing an electrodeposition solution, immersing aconductive substrate in the electrodeposition solution andelectrodepositing a zinc ferrite thin film on the conductive substrateunder suitable parameters, and drying the zinc ferrite thin film with adrying temperature of about 15-40° C. and a relative humidity of atleast about 75%.

Said electrodeposition solution comprises about 10⁻² M to about 10⁻¹ Mzinc nitrate aqueous solution and about 10⁻³ M to about 10⁻² M ferricnitrate aqueous solution

Said suitable parameters include: an Ag/AgCl reference electrode; anelectrodeposition voltage of about 900-1100 mV; and a working distanceof about 1-5 cm.

In further embodiments of the present invention, the zinc ferrite thinfilm obtained by the above-mentioned procedures can undergo an annealingstep.

In another aspect, the present invention is directed to applications ofsaid zinc ferrite thin film. For example, said zinc ferrite thin filmcan be used as a photoelectrode of a photosensitized solar cell. Inaddition, since said zinc ferrite thin film possesses the photo-electroconversion ability, zinc ferrite thin film can also act as a solid-statesensitizing layer of the photosensitized solar cell to partially orcompletely substitute for the dye layer of a the photosensitized solarcell.

According to one embodiment of the present invention, the photoelectrodeof a photosensitized solar cell comprises a conductive substrate and azinc ferrite thin film located on a surface of the conductive substrate.

In yet another aspect, the present invention is directed to aphotosensitized solar cell that utilizes said zinc ferrite thin film asa photoelectrode and/or a solid-state sensitizing layer.

According to one embodiment of the present invention, thephotosensitized solar cell comprises a photoelectrode, an electrolyteand a counter electrode laminated in this order. Said photoelectrodecomprises a conductive substrate and a zinc ferrite thin film located ona surface of the conductive substrate, wherein said surface faces thecounter electrode.

Many of the attendant features will be more readily appreciated as thesame becomes better understood by reference to the following detaileddescription considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present description will be better understood from the followingdetailed description read in light of the accompanying drawings,wherein:

FIG. 1 is an X-ray diffraction pattern of a zinc ferrite thin filmaccording to one embodiment of the present invention; and

FIG. 2 is a UV-Vis absorption spectrum of a zinc ferrite thin filmaccording to one embodiment of the present invention and a commerciallyavailable N₃ dye.

DETAILED DESCRIPTION

The detailed description provided below in connection with the appendeddrawings is intended as a description of the present embodiments and isnot intended to represent the only forms in which the presentembodiments may be constructed or utilized. The description sets forththe functions of the embodiments and the sequence of steps forconstructing and operating the embodiments. However, the same orequivalent functions and sequences may be accomplished by differentembodiments.

(I) Electrochemical Methods for Manufacturing Zinc Ferrite Thin Film

In one aspect, the present invention is directed to electrochemicalmethods for manufacturing a zinc ferrite thin film. For the purpose ofillustration but not restriction, zinc-containing salt andiron-containing salt in the electrodeposition solution would dissociatesto produce zinc ions and iron ions, said zinc ions and iron ions formcomplex ions with water molecules, and said complex ions would depositon the surface of a conductive substrate (working substrate).

According to embodiments of the present invention, the electrochemicalmethod for manufacturing zinc ferrite thin film includes the procedureof preparing an electrodeposition solution, immersing a conductivesubstrate in the electrodeposition solution and electrodepositing thezinc ferrite thin film on the conductive substrate under suitableparameters, and drying the zinc ferrite thin film.

According to embodiments of the present invention, saidelectrodeposition solution comprises about 10⁻² M to about 10⁻¹ M zincnitrate aqueous solution and about 10⁻³ M to about 10⁻² M ferric nitrateaqueous solution.

According to embodiments of the present invention, said step ofelectrodeposition is carried out at room temperature (about 23-27° C.).Besides, the electrodeposition solution can be stirred during theelectrodeposition process, and the zinc ion and iron ion should betimely supplemented to maintain required concentration thereof.

According to embodiments of the present invention, parameters forelectrodeposition of zinc ferrite thin film include: an Ag/AgClreference electrode; a platinum counter electrode; an electrodepositionvoltage of about 900-1100 mV; an electrodeposition time about 15-120minutes, and a working distance of about 1-5 cm.

According to embodiments of the present invention, said conductivesubstrate can be a conductive fabric, a transparent conductivesubstrate, a metal substrate, or a metal oxide substrate. As an example,but not as a limitation, the conductive fabric can be made of conjugatedpolymers or made from metallic fibers/yarns; the transparent conductivesubstrate can be a fluorine-doped tin oxide/glass (FTO/glass) substrate,an indium tin oxide/glass (ITO/glass) substrate, or an ITO/polyethylenenaphthalate (ITO/PEN) flexible substrate; and metal substrate can be aplatinum substrate or a stainless steel substrate.

According to embodiments of the present invention, suitable dryingtemperature is about 15-40° C. and relative humidity is at least about75%. In some preparation examples of the present invention, the zincferrite thin film was dried in a constant temperature and humidityapparatus, and the drying temperature used was about 30° C. and therelative humidity used was about 80%.

Furthermore, according to other embodiments of the present invention,the zinc ferrite thin film can be heat-treated at about 150-450° C.after the drying step. The heat treating step can be carried on severalstages. For example, the heating step can include a first heating stage,a second heating stage, and a cooling stage. Specifically, in the firstheating stage, the zinc ferrite thin film is heated from 23-27° C. toabout 70-100° C. at a first heating rate of about 2° C. per minute forabout 30-120 minutes; in the second heating stage, the zinc ferrite thinfilm is further heated to about 150-450° C. at a second heating rate ofabout 2° C. per minute for about 60-120 minutes; and in the coolingstage, the zinc ferrite thin film is cooled to about 23-27° C. at acooling rate of about 2° C. per minute.

As will occur to those skilled in the art, the temperature ofheat-treatment depends on the conductive substrate used. For example,with respect to plastic substrates, the temperature of heat-treatmentshould not exceed 300° C. and preferably should not exceed 150° C.

In the following preparation examples, some processing parameters werealtered according to the embodiments of the present invention tomanufacture zinc ferrite thin films. The altered parameter(s) of eachexample are indicated in Table 1. All examples listed in Table 1 weredried with a drying temperature of about 40° C. and a relative humidityof about 85%. In Examples 1-8, FTO/glass was used as the workingelectrode; while in Examples 9 and 10, ITO/PEN was used as the workingelectrode.

TABLE 1 Volt- Working Heat-treating Zn(NO₃)₂ Fe(NO₃)₃ age Time DistanceTemperature (M) (M) (mV) (min) (cm) (° C.) Exam- 10⁻¹ 10⁻² 1100 15 1 150ple 1 Exam- 10⁻¹ 10⁻² 1100 15 1 300 ple 2 Exam- 10⁻¹ 10⁻² 1100 15 1 450ple 3 Exam- 10⁻¹ 10⁻² 900 15 1 300 ple 4 Exam- 10⁻¹ 10⁻² 1100 30 1 300ple 5 Exam- 10⁻¹ 10⁻² 1100 60 1 300 ple 6 Exam- 10⁻² 10⁻³ 1100 30 1 300ple 7 Exam- 10⁻¹ 10⁻³ 1100 120 1 300 ple 8 Exam- 10⁻¹ 10⁻² 1100 15 1 150ple 9 Exam- 10⁻¹ 10⁻² 1100 15 5 150 ple 10

In Examples 1-10, zinc ferrite thin films could be formed on the workingsubstrates after the drying step. The zinc ferrite thin films thusobtained were further heat treated to substantially remove residualwater in the zinc ferrite thin films.

(II) Property Analysis of Zinc Ferrite Thin Films

1. X-ray Diffraction Analysis

X-ray diffraction pattern of the zinc ferrite thin film was obtained byX-ray diffractometer (Model: MAC MO3X—HF Diffractometer) with thefollowing settings: Kα radiation λ=1.5418; scan range 2Θ=15-70′; scanrate: 1°/min; voltage: 40 kV; and generator current: 30 mA. FIG. 1 is anX-ray diffraction pattern of the zinc ferrite thin film of Example 1. InFIG. 1, diffraction peaks of zinc ferrite were marked with ★. Thepattern shown in FIG. 1 confirms that zinc ferrite thin film could bemanufactured by the methods mentioned above.

2. UV-Vis Absorption Analysis

As stated above, nano-sized zinc ferrite is capable of absorbing visiblelight and possesses photo-electro conversion ability; hence, the zincferrite thin film according to the embodiments of the present inventioncan be used as the solid-state sensitizing layer to partially orcompletely substitute for the dye layer of a the photosensitized solarcell.

The absorption spectrum pattern of the zinc ferrite thin film wasobtained by a spectrometer (Model: Hitachi U-4100). FIG. 2 is a UV-Visabsorption spectrum of a zinc ferrite thin film of Example 1 and acommercially available N₃ dye. The absorption spectrum shown in FIG. 2suggests that the zinc ferrite thin film has absorption of visible lightin the range of about 400-650 nm.

(III) Manufacture of Photoelectrodes and Photosensitized Solar Cells

In another aspect, the present invention is directed to applications ofsaid zinc ferrite thin film. For example, said zinc ferrite thin filmcan be used as a photoelectrode of a photosensitized solar cell.

According to one embodiment of the present invention, the photoelectrodeof a photosensitized solar cell may comprise a conductive substrate anda zinc ferrite thin film located on a surface of the conductivesubstrate.

According to embodiments of the present invention, the photoelectrode ofa photosensitized solar cell may further comprise a conductive materiallocated at said surface of the conductive substrate. Said conductivematerial could be titanium oxide, zinc oxide, copper oxide, zirconia, ora combination thereof.

For the purpose of illustration but not restriction, a zinc ferrite thinfilm can be formed on a surface of the conductive substrate according tothe embodiment of the present invention, and then the conductivematerial can be formed on the same surface. Or, the conductive materialcan be formed on a surface of the conductive substrate, and then thezinc ferrite thin film can be formed on a surface of the conductivesubstrate according to the embodiment of the present invention. Or, thezinc ferrite and the conductive material can be formed on one surface ofthe conductive surface at the same time.

In addition, since said zinc ferrite thin film possesses thephoto-electro conversion ability, zinc ferrite thin film can also act asa solid-state sensitizing layer of the photosensitized solar cell topartially or completely substitute for the dye layer of a thephotosensitized solar cell. Thus, in yet another aspect, the presentinvention is directed to a photosensitized solar cell that utilizes saidzinc ferrite thin film as a photoelectrode and/or a solid-statesensitizing layer.

According to one embodiment of the present invention, thephotosensitized solar cell comprises a photoelectrode, an electrolyteand a counter electrode laminated in this order. Said photoelectrodecomprises a conductive substrate and a zinc ferrite thin film located ona surface of the conductive substrate, wherein said surface faces thecounter electrode.

According to embodiments of the present invention, any suitableelectrolyte can be filled/formed between the photoelectrode and thecounter electrode. For the purpose of illustration but not restriction,said electrolyte can be an acetonitrile solution comprising about 0.5 Mlithium iodide and about 0.05 M iodine. Specifically, in the preparationexamples of the present invention, the electrolyte is an acetonitrilesolution containing about 0.5 M M lithium iodide, about 0.05 M iodineand about 0.5 M 4-tert-butylpyridine.

Though the zinc ferrite thin film according to embodiments of thepresent invention possesses photo-electro conversion ability which canbe used as the solid-state sensitizing layer photosensitized solar cell,materials can be photosensitized such as dye may be additionally usedwhen manufacturing the photosensitized solar cell to enhance theefficiency of the solar cell.

Ruthenium complex is a commonly used photo-sensitized dye. Saidruthenium complex includes but is not limited to N₃ dye(cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)), N₇₁₂ dye((Bu₄N)₄[Ru (dcbpy)₂(NCS)₂] Complex), N₇₁₉ dye(cis-bis(isothiocyanato)-bis-(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bis(tetrabutylammonium))and N₇₄₉ dye((2,2′:6′,2-terpyridine-4,4′,4-tricarboxylate)ruthenium(II)tris(tetrabutylammonium)tris(isothiocyanate)), etc. However, the dyes listed above are only forexamples, and as will occur to those skilled in the art, any othersuitable dyes may be used in the photosensitized solar cell ofembodiments of the present invention.

In the following preparation examples, zinc ferrite thin films of theExamples 1-10 were used to prepare photosensitized solar cells, andphotoelectrochemical analysis was conducted to understand thephoto-electro conversion efficiency and other current-voltagecharacteristics of each photosensitized solar cell and the results arelisted in Table 2.

The photosensitized solar cell of the preparation example comprises aphotoelectrode according to embodiments of the present invention; aplatinum counter electrode; and an electrolyte of an acetonitrilesolution containing about 0.5 M lithium iodide, about 0.05 M iodine andabout 0.5 M 4-tert-butylpyridine. In the following description, PSC 1refers to the photosensitized solar cell that uses zinc ferrite thinfilm of the Example 1 as a photoelectrode; similarly, PSC 3 refers tothe photosensitized solar cell that uses zinc ferrite thin film of theExample 3 as a photoelectrode. Furthermore, among the preparationexamples listed in Table 2, PSC 8 is the only photosensitized solar cellthat used an N₃ dye, and the other photosensitized solar cells did notuse additional dye in the solid-state sensitizing layer. The surface ofthe photoelectrode of PSC 8 was attached with the N₃ dye by conventionalmethods.

In the photoelectrochemical analysis, the photosensitized solar cell wasirradiated with a light of 1000 W/m² using a solar simulator of AM 1.5as a light source, and current-voltage characteristics were measured andcalculated. The current-voltage characteristics include short circuitcurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and solarenergy to electricity conversion efficiency (η) of the photosensitizedsolar cell of the present invention.

TABLE 2 Voc(mV) Jsc (mA/cm²) FF η (%) PSC 1 240 0.21 0.37 0.02 PSC 2 2100.49 0.33 0.03 PSC 3 320 0.71 0.36 0.08 PSC 4 170 0.35 0.32 0.02 PSC 5180 0.82 0.31 0.05 PSC 6 310 0.21 0.31 0.02 PSC 8 350 0.32 0.47 0.05 PSC9 270 0.07 0.40 0.01 PSC 10 300 0.01 0.51 0.0014

As can be seen in Table 2, the change of process parameters of theelectrochemical method for manufacturing the zinc ferrite thin filmwould affect the current-voltage characteristics of the photosensitizedsolar cell. Specifically, with respect to the heat-treating temperatureof the zinc ferric thin film, it can be appreciated from PSC 1 (aheat-treating temperature of which is about 150° C.), PSC 2 (aheat-treating temperature of which is about 300° C.) and PSC 3 (aheat-treating temperature of which is about 450° C.) that as theheat-treating temperature increases from about 150° C. to about 450° C.,the photo-electro conversion efficiency also increases from about 0.02%to about 0.08%.

Furthermore, with respect to the electrodeposition voltage formanufacturing the zinc ferric thin film, it can be appreciated from PSC2 (an electrodeposition voltage of which is about 1100 mV) and PSC 4 (anelectrodeposition voltage is about 900 mV) that the open circuitvoltage, short circuit current, fill factor and photo-electro conversionefficiency of the photosensitized solar cell increase as theelectrodeposition voltage increases.

With respect to the electrodeposition time of the zinc ferric thin film,it can be seen from PSC 2 (an electrodeposition time of which is about15 minutes), PSC 5 (an electrodeposition time of which is about 30minutes) and PSC 6 (an electrodeposition time of which is about 60minutes) that electrodeposition time also affects the current-voltagecharacteristics of the photosensitized solar cell. It can be seen inTable 2 that the short circuit current and photo-electro conversionefficiency of PSC 5 are superior to that of PSC 2 and of PSC 6.

With other parameters remain unchanged, the material of the workingelectrode would also affect the current-voltage characteristics of thephotosensitized solar cell. For example, PSC 1 used FTO/glass as theworking electrode, while PSC 9 used ITO/PEN as the working electrode,and it can be seen in Table 2 that the short circuit current andphoto-electro conversion efficiency of PSC 1 are superior to the same ofPSC 9; while the open circuit voltage and fill factor of PSC 9 aresuperior to the same of PSC 1.

The working distance for manufacturing the zinc ferric thin film is theonly difference between PSC 9 (a working distance of which is about 1cm) and PSC 10 (a working distance of which is about 5 cm). The resultsshown in Table 2 suggest that the short circuit current andphoto-electro conversion efficiency of PSC 9 are superior to that of PSC10, while the open circuit voltage and fill factor of PSC 10 aresuperior to that of PSC 9.

As previously stated, PSC 8 is the only photosensitized solar cell thatused an N₃ dye, and the other photosensitized solar cells did not useadditional dye in the solid-state sensitizing layer. The results inTable 2 show that the open circuit voltage of PSC 8 is the highest amongthe listed examples, however, the short circuit currents of PSCs 2-5 arehigher than that of PSC 8, and the fill factor of PSC 10 is also higherthan that of PSC 8. Furthermore, the photo-electro conversion efficiencyof PSC 3 is superior to that of PSC 8, while the same of PSC 5 is aboutthe same as that of PSC 8. From the results shown in Table 2, it can beappreciated that the zinc ferrite thin film of the embodiments of thepresent invention can be used as the photoelectrode and as thesolid-state sensitizing layer of the photosensitized solar cell topartially or completely substitute for the dye layer of a thephotosensitized solar cell.

From the results and discussion described above, it is concluded thatzinc ferrite thin film with different current-voltage characteristics bycontrolling the process parameters for manufacturing the zinc ferritethin film according to the embodiments of the present invention; and thezinc ferrite thin films thus obtained possess desirable photo-electroconversion ability and may be useful in further industrial application.

It will be understood that the above description of embodiments is givenby way of example only and that various modifications may be made bythose with ordinary skill in the art. The above specification, examplesand data provide a complete description of the structure and use ofexemplary embodiments of the invention. Although various embodiments ofthe invention have been described above with a certain degree ofparticularity, or with reference to one or more individual embodiments,those with ordinary skill in the art could make numerous alterations tothe disclosed embodiments without departing from the spirit or scope ofthis invention.

1. A photoelectrode of a photosensitized solar cell, comprising: aconductive substrate; and a zinc ferrite thin film located on a surfaceof the conductive substrate.
 2. The photoelectrode of a photosensitizedsolar cell of claim 1, wherein the photoelectrode further comprises aconductive material located on the surface of the conductive substrate,the conductive material is selected from the group consisting oftitanium oxide, zinc oxide, copper oxide, zirconia, and a combinationthereof.
 3. A photosensitized solar cell, comprising sequentially aphotoelectrode, an electrolyte and a counter electrode, wherein thephotoelectrode comprises a conductive substrate and a zinc ferrite thinfilm located on a surface of the conductive substrate that faces thecounter electrode.
 4. The photosensitized solar cell of claim 3, furthercomprising a sensitized dye attached to the zinc ferrite thin film ofthe conductive substrate.
 5. The photosensitized solar cell of claim 3,wherein the electrolyte is an acetonitrile solution comprising about 0.5M lithium iodide, about 0.05 M iodine and about 0.5 M4-tert-butylpyridine.
 6. The photosensitized solar cell of claim 3,wherein the photoelectrode further comprising a conductive materiallocated on the surface of the conductive substrate, the conductivematerial is selected from the group consisting of titanium oxide, zincoxide, copper oxide, zirconia, and a combination thereof.